It is known to operate two discharge lamps with two load circuits. In this case, the term load circuit refers to the load of a bridge that is used as an inverter to operate a discharge lamp. Each load circuit has a dedicated preheating arrangement for the respective lamp. Furthermore, according to the internal prior art, it is possible to operate two lamps in one load circuit. Here, the primary coil of a heating transformer of a series circuit of two lamps is connected in parallel and the secondary coil of the heating transformer is connected between the two lamps. Furthermore, it is possible to heat all the filaments of the lamps by transformer via secondary windings, the primary winding being situated in a section of the bridge suitable for the application.
It is relatively complicated to implement the load circuits in terms of circuitry, since electronic control circuits with relay or transistor switches are required for a defined, sequential starting and subsequent joint operation of the lamps. By contrast, relatively favorable control circuits that use only passive components for controlling the preheating exist for the purpose of operating individual lamps. The essential constituent of such circuits is a heat-sensitive resistor with a positive temperature coefficient.
A bridge circuit with a relevant load circuit is illustrated in FIG. 1. The bridge is implemented for the purpose of inversion as a half bridge with two switching elements 1 and 2 and two capacitors 3 and 4. The load circuit 5 in the bridge comprises a coil 6 in series with a lamp 7 which is connected in parallel both with a resonance capacitor 8 and with a heat-sensitive resistor 9.
The mode of operation of the circuit illustrated in FIG. 1 may be explained as follows. By actuating the switches 1 and 2 suitably, an AC voltage for the load circuit 5 is generated in the center tap of the bridge from the DC voltage. The frequency of the AC voltage is advantageously in the region of the resonant frequency of the coil 6 and the capacitor 8 for the ignition process of the lamp. Before the ignition, as (PTC) thermistor the resistor 9 with a positive temperature coefficient (PTC) detunes the series resonant circuit 6, 8 in such a way that the required ignition voltage across the lamp 7 or the capacitor 8 is not reached. However, the current is already flowing through the incandescent filaments 10 and 11 of the lamp 7 such that they are preheated for the ignition process. In the meantime, current is likewise flowing through the PTC thermistor 9, which it heats in this preheating phase. Its resistance rises in the process, and so the detuning of the series resonant circuit, 6, 8 is correspondingly reduced such that the ignition voltage across the lamp 7 can be reached. The PTC thermistor 9 is designed in this case such that it carries a sufficient quantity of current even after ignition in order to remain highly resistant so that the resonance can be maintained at an appropriate level of quality.
For the sake of clarity, the load circuit 5 is illustrated in FIG. 2a without the coil 6. FIG. 2b shows a variant of the load circuit of FIG. 2a. Connected in series with the PTC thermistor 9 is a series capacitor 12 which has the effect that the detuning of the resonant circuit by the PTC thermistor 9 is not so marked as in the case of the circuit of FIG. 2a. This means that in this case the ignition voltage is reached more quickly and the lamp is ignited more rapidly as a consequence thereof.
A further variant of the load circuits that are illustrated in FIGS. 2a and 2b is reproduced in 2c. In this case, the series capacitor 12 is chiefly active in the cold state of the PTC thermistor 9, whereas the series circuit of the two capacitors 8 and 9 is only active in the warm state of the PTC thermistor 9, that is to say during the operation and ignition of the lamp.